1. Field of the Invention
The present invention relates to a device and a method for treating an exhaust gas containing particles. The device comprises an exhaust gas line, an emission electrode, and an electrical supply line for the electrical contacting of the emission electrode. The electrical supply line is enclosed in sections by an electrical insulator, which electrically insulates the electrical supply line in relation to the exhaust gas line. The electrical insulator comprises a surface against which the exhaust gas can flow.
2. Related Art
In motor vehicles having mobile internal combustion engines and in particular in motor vehicles having diesel drive, quantities of soot particles are regularly contained in the exhaust gas of the internal combustion engine, which cannot be emitted into the environment. This is specified by corresponding exhaust gas regulations, which specify limiting values for the number and the mass of soot particles per exhaust gas weight or exhaust gas volume and sometimes also for an entire vehicle. Soot particles are in particular non-combusted carbons and hydrocarbons in the exhaust gas.
A variety of different concepts for removing soot particles from exhaust gases of mobile internal combustion engines have already been discussed. In addition to mutually closed wall-flow filters, open secondary flow filters, gravity separators, etc., systems have also already been proposed in which the particles in the exhaust gas are electrically charged and then deposited with the aid of electrostatic attractive forces. These systems are known in particular under the name “electrostatic filter” or “electrofilter”.
In “electrofilters”, an agglomeration of small soot particles to form larger soot particles and/or an electrical charge in soot particles is caused by providing an electrical field and/or a plasma. Electrically charged soot particles and/or larger soot particles are generally substantially simpler to separate in a filter system. Soot particle agglomerates are transported more slowly in an exhaust gas flow because of their greater mass inertia and therefore accumulate more easily at deflection points of an exhaust gas flow. Electrically charged soot particles are drawn, as a result of their charge, toward surfaces, on which they accumulate and dissipate their charge. This also makes it easier to remove soot particles from the exhaust gas stream in operation of motor vehicles.
Generally multiple discharge electrodes and collector electrodes, which are positioned in the exhaust gas line, are proposed for such electrofilters. In this case, for example, a central discharge electrode, which extends approximately centrally through the exhaust gas line, and a surrounding lateral surface of the exhaust gas line, as a collector electrode, are used to form a capacitor. Using this arrangement of the discharge electrode and the collector electrode, an electrical field is formed transversely to the flow direction of the exhaust gas, wherein the discharge electrode can be operated using a high voltage, for example, which is in the range of approximately 15 kV. In this way, corona discharges can form in particular, by which the particles flowing with the exhaust gas through the electrical field are charged in a unipolar manner. As a result of this charging, the particles travel toward the collector electrode due to the electrostatic Coulomb forces.
In addition to systems in which the exhaust gas line is embodied as the collector electrode, systems are also known in which the collector electrode is formed as a wire lattice, for example, in this case particles accumulate on the wire lattice for the purpose of possibly bringing together the particles with further particles, to thus implement an agglomeration. The exhaust gas flowing through the lattice then entrains the larger particles again and supplies them to classical filter systems.
In the case of the regeneration of filter systems, in addition to intermittent regeneration by way of short-term heating, i.e., combustion of the soot (catalytically motivated, oxidative reaction), converting soot by nitrogen dioxide (NO2) is also known. The advantage of continuous regeneration using nitrogen dioxide (CRT method) is that the conversion of soot can already take place here at significantly lower temperatures (in particular less than 250° C.). For this reason, continuous regeneration is preferred in many applications. However, this results in the problem that it must be ensured that the nitrogen dioxide in the exhaust gas comes into contact with the deposited soot particles to a sufficient extent.
Technical difficulties also result in this context in the implementation of continuous operation of such exhaust gas systems in motor vehicles, wherein the differing loads of the internal combustion engines result in different exhaust gas streams, exhaust gas compositions, and/or temperatures.
In addition, it is to be taken into consideration that the simplest possible components are to be used when providing such components for such a soot separating system, in particular also those components which may be produced cost-effectively in the scope of mass production. In addition, in particular in the design of the electrodes, it is to be taken into consideration that they must be positioned aligned in the exhaust gas line if necessary, in particular so that an undesired high stagnation pressure or undesired swirling of the exhaust gas does not occur in the region of the electrode.
Although the above-described systems have heretofore proven to be suitable for the treatment of soot particles at least in experiments, the implementation of this concept for mass production in motor vehicles nonetheless represents a substantial challenge. Soot particles accumulate in particular on the electrical insulation of the electrode and the counter electrode in relation to the exhaust gas line.
If a continuous soot deposit has formed from the electrode to the exhaust gas line via the electrical insulation, an electrical current (“leakage current”) thus flows through it. Preventing a continuous soot accumulation has therefore previously been attempted. Thus, for example, introducing a gas via the electrical insulation into the exhaust gas line has been proposed, so that the soot particles are deflected away from the electrical insulation by the gas. Using targeted deflection devices for the exhaust gas, which deflect the exhaust gas from the electrical insulation, has also already been proposed.
Additionally or alternatively, designing the electrical insulation having the largest possible surface has previously been attempted, so that the formation of a continuous soot deposit becomes less probable. To enlarge the surface, the electrical insulators are provided with ribs extending longitudinally or transversely, for example.
To remove the soot deposits, intentionally increasing the temperature of the electrical insulator, so that the soot deposits burn off, has additionally already been proposed. For this purpose, it has been proposed, for example, that an electrical conductor be formed in the electrical insulator, which can elevate the temperature of the electrical insulator. Coating the electrical insulator using a catalytically active substance is also known.
However, it has been shown that in spite of all of these efforts, a continuous soot deposit on the electrical insulation between electrode and exhaust gas line cannot be prevented in operation of a motor vehicle.